The invention relates to sewing machines in general, and more particularly to improvements in apparatus for controlling the operation of certain components in sewing machines, especially the operation of a variable speed motor.
In many instances, the speed of a variable-speed motor, particularly an electric motor, in a sewing machine is controlled by a foot-operated device which includes a pedal or treadle. This enables the operator of the sewing machine to select the number of stitches per unit of time. As a rule, the foot-operated device comprises a switch, a series of switches, a variable resistor or a like part which must be connected to electric conductors. Reference may be had, for example, to Swiss Pat. No. 659,054 wherein a pedal must be pivoted in a first direction in order to actuate a variable resistor which initiates the operation of the electric motor at a progressively increasing speed. If the pedal is pivoted in a second direction counter to the first direction, it completes the circuit of a device which causes the needle of the sewing machine to assume its upper or lower end position when the motor is brought to a halt. A drawback of the apparatus which is disclosed in the Swiss patent is that electric current must flow through the foot-operated device which contributes to the cost, particularly for proper insulation, brings about the danger of electric shock to the operator of the sewing machine, and renders it necessary to suppress potential interference.
U.S. Pat. No. 4,310,788 to Hanyu et al. discloses a speed control device for sewing machines wherein a triangular light beam interrupting member is movable in a foot-operated device to control the extent of penetration of light from a first set of optical fibers into a second set of optical fibers. The amount of light which penetrates into the second set of optical fibers determines the speed of the electric motor of the sewing machine. A drawback of the patented proposal is that the speed of the motor cannot be regulated with a requisite degree of accuracy. More specifically, it is difficult to ensure gradual (rather than jerky stepwise) variation of the speed of the motor and hence the number of stitches per unit of time. Furthermore, it is normally necessary to utilize a rather large number of optical fibers, the light interrupting member must be finished with a high degree of precision, and the means for moving the light interrupting member must be assembled with a very high degree of accuracy. This cannot be readily achieved in view of the normally large manufacturing tolerances for light emitting and light receiving components of presently known and available optical detectors. Moreover, the patented apparatus is designed to control only the speed of the electric motor in the sewing machine.